Electronic key or other missing object finder devices that produce an audible alert signal when the device senses a predetermined triggering signal are known in the art. The heretofore known devices may be divided into "two-unit" and "single-unit" devices. U.S. Pat. No. 4,101,873 to Anderson is representative of the heretofore known two-unit electronic key or other missing object locator devices and U.S. Pat. No. 4,507,653 to Bayer is representative of the heretofore known single-unit devices.
For the two-unit devices, one of the units is always a special-purpose transmitter that provides a predetermined triggering signal, while the other is a dedicated receiver responsive thereto to sound an audible alert. In Anderson, for example, the predetermined triggering signal is provided in practice by a modulated carrier where the modulation is a number of pre-selected single frequency tones. To the extent that the predetermined triggering signals provided by the heretofore known two-unit devices are uniquely unlike the kinds of signals that would otherwise be present in a typical use environment, they generally exhibit good noise immunity and freedom from false activation but are subject to the disutility that the special-purpose transmitters thereof are no less likely to be misplaced than the missing objects to be located thereby.
For the single-unit devices, the predetermined triggering signals are generated by the user who claps, blows a whistle, or otherwise "humanly produces" the intended triggering sounds that activate the device. Although the single-unit electronic key or other missing object finder devices thereby eliminate the problem of a "lost transmitter," the heretofore known devices are highly prone to unintended activation by a plethora of sounds that may be deceptively similar to the intended triggering sounds, such as would be generated by such ambient noise sources as radios, televisions, loud voices and background music, as well as by mechanical vibrations, among other things.
In Bayer, for example, a single-unit device is disclosed that is responsive to a plurality of human-generated intended triggering sounds for emitting audible missing object locator tones. In the illustrated embodiment thereof, the audible tones are produced thereby whenever the user generates four (4) triggering sounds by clapping his or her hands in such way as to temporally order each clap a minimum of eleven-sixteenths (0.6875) seconds apart within four (4) seconds (the "intended plurality of triggering sounds").
A training circuit having an LED that is "turned-off" by hand claps is disclosed to allow the user to learn to space the four required claps in the required time interval and with the required spacing during operation in a "training" mode.
Bayer discloses a signal discrimination technique that is based solely on whether or not the received sounds exhibit a temporal order that corresponds to the temporal order of the intended plurality of triggering sounds to determine whether sounds received by the single-unit missing object locator thereof have been produced by a user or not. If the temporal order of the plurality of actually received sounds does correspond to the temporal order of the intended plurality of triggering sounds, then the audible alert is produced by the Bayer signal discrimination technique; otherwise, it is not. Bayer also discloses a false alarm rejection technique that likewise is based solely on the temporal order of the received sounds. If the temporal order of the plurality of actually received sounds does not correspond to the temporal order of the intended plurality of triggering sounds, then they are rejected as "false," as they are not in accord with the temporal order expected by the corresponding Bayer signal discrimination technique.
The false alarm rejection technique of Bayer rejects noise for no more than a fixed, predetermined time (the so-called "fourth predetermined time"), during which the unit's ability to detect triggering sounds is deactivated. A motion detector is provided which deactivates the unit's ability to detect triggering sounds for no more than the same fixed, predetermined time, whenever the unit's motion itself gives rise to noise.
However, as will readily be appreciated by those of skill in the art, the signal discrimination and false alarm rejection techniques based on the temporal order of received sounds of the Bayer single-unit device are disadvantageous in that they allow the Bayer device to provide an audible output for any four (4) sounds which may happen to occur within the expected temporal order irregardless of whether they arise from an intended human-generated hand clap or from ambient noise such as speech, music and the rest. The audible alert is provided for any received triggering sounds, so long as they exhibit the expected temporal order. Because the Bayer single-unit missing object locator device responds to both the intended triggering sounds as well as to ambient noise, it is subject to annoying false activation by music, speech, or any ambient sources other than the intended hand claps, which renders it generally inoperable in noisy environments and makes it prone to false alarms in day to day use.
Moreover, both the Bayer false alarm rejection technique and motion detection technique that reject noise for no more than a predetermined time give rise to power-draining toggling of the unit "on" and "off" in environments with on-going noise sources, as the unit's ability to detect triggering sounds is deactivated, then activated, and so on repetitively, by the on-going environmental noise. In addition, the Bayer motion detection technique is only effective whenever the unit itself is in motion, but does not detect possible noise conditions when the unit is itself unmoved.